Add support for contain property in CSS compiler.

See:
https://developers.google.com/web/updates/2016/06/css-containment?hl=en
https://drafts.csswg.org/css-containment/
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=125691734

src/com/google/common/css/compiler/ast/Property.java

@@ -153,6 +153,7 @@ public final class Property {
         builder("column-span"),
         builder("column-width"),
         builder("columns"),
+        builder("contain"),
         builder("content"),
         builder("counter-increment"),
         builder("counter-reset"),